Platelet function is of fundamental importance in the development of arterial thrombosis and cardiovascular diseases. Nowadays it is a matter of course that patients suffering from cardiovascular diseases are treated with antiplatelet drugs. Despite the availability of various clinically successful antiplatelet therapies, there is still a large unmet medical need for new treatments. This deficiency is mainly caused by the limited efficacy of the currently available drugs, particularly in regard to the drugs efficacy-safety (bleeding) correlation. Interfering with early events of platelet activation and adhesion, a mechanism not targeted by drugs currently in use, would be an attractive approach for the improvement of the efficacy-safety margin. The collagen receptor glycoprotein (GPVI) is of central importance in these early events of platelet activation, and therefore a major target for the interference with this mechanism (Nieswandt B and Watson S P, Blood. 2003 Jul. 15; 102(2):449-61). The antiplatelet and antithrombotic effects of GPVI have been described in several in vitro and in vivo systems, using platelets from mice and men. Platelets deficient in GPVI are rendered unresponsive to collagen, one of the most important thrombogenic components of the subendothelial matrix (Lockyer S. et al, Thromb Res. 2006; 118(3):371-80). Moreover, mouse studies have shown that GPVI deficiency causes an effective inhibition of arterial thrombus formation at the damaged vessel wall without increasing the susceptibility to spontaneous bleeding. All these data suggest that GPVI represents an effective and safe target for the treatment of arterial thrombosis. The central role of GPVI in the initiation of thrombus formation indicates that inhibition of this receptor may be beneficial in syndromes of arterial thrombosis. This makes the use of biotherapeutic proteins such as antibodies a clinically meaningful strategy for the inhibition of GPVI. All the more since the interaction of GPVI and its ligand collagen seems to involve an expanded protein surface, a successful interference with this protein-protein interaction is more likely with inhibitory GPVI-binding proteins as compared to other strategies. An in vivo proof of concept for the inhibition of GPVI function by antibodies and Fab fragments has been shown in several animal models.
There is still a need for clinically effective inhibitors of GPVI activity.
GPVI is a major collagen receptor expressed exclusively on platelets and megacaryocytes. Binding to collagen induces receptor clustering and subsequent platelet activation. This is one of the initial events in thrombus formation. Current anti-platelet drugs interfere with thrombus formation through targeting late events in this process. A serious side effect of these drugs is prolonged bleeding which limits their use. There are several lines of evidence that targeting early events in thrombus formation (such as GPVI) can be highly antithrombotic without a major impact on bleeding liability. The interaction between collagen and GPVI can be successfully inhibited by neutralizing monoclonal antibodies (mAb). However as mAb's are bivalent molecules, they can induce GPVI-receptor clustering and therefore platelet activation. To circumvent this, monovalent antibody fragments such as Fab fragments have been developed.
Unfortunately, in depth safety profiling of these monovalent anti-GPVI Fab fragments reveals a potential to still induce platelet activation in a patient specific manner. To offer a safe therapeutic for the treatment of ischemic events, this activatory potential of the developed Fab molecules had to be abolished. Until now, this problem has not been resolved.